Production of bundles, fabrics, felts, etc. of carbon fibers has been conducted by continuously carbonizing precursors that have been previously rendered heat resistant, in an inert gas at a temperature above 500.degree. C. Many apparatuses for production have a tubular heating chamber providing a path through which the precursor is carried while being heated.
Apparatuses for continuously producing carbon fibers are known in which a carbon fiber precursor passes through a tubular heating chamber which is vertically set and externally heated, as described in Japanese patent application (OPI) Nos. 116224/76 (corresponding to U.S. Pat. Nos. 4,073,870) and 119834/76 (the term "OPI" as used herein means an "unexamined published Japanese Patent application"). Heaters commonly used in this type of apparatus include a resistive heating system and an induction heating system.
In these carbonizing furnaces, alumina fiber felt, ceramic felt, carbon black or carbon particles have been widely employed as heat insulating materials (heat insulators).
Such conventional carbonizing furnaces have the following problems.
(1) Carbon black or carbon particles, when used as a heat insulator around a heating chamber, can form a conductive path across electrical insulation, which leads to a breakdown.
(2) The carbon black and carbon particles are difficult to handle.
(3) Alumina/silica type ceramic fiber felt which has heat resistance (for using at least six months continuously) at a temperature not higher than, for example, 1500.degree. C. cannot be used as a heat insulator for carbonizing furnaces where heat treatment is effected at temperatures higher than 1800.degree. C. because fibers gradually shrink.
(4) Carbon fibers coiled around a heating chamber as a heat insulator exhibit higher heat resistance than the ceramic fiber felt when heat treatment is effected at 1800.degree. C. or higher. However, carbon fibers are difficult to handle and very expensive.
(5) Carbon felt, when used as a heat insulator in a conventional carbonizing furnace, scatters short fibers that can form an electrically conductive path, and cause shortcircuiting.